A photoacoustic imaging technique is known as one of imaging techniques using light. In the photo acoustic imaging, first, pulsed light is generated by a light source and a subject is illuminated with the pulsed light. In the subject, the illumination light propagates while diffusing. Energy of light is absorbed by a plurality of parts in the subject, which may cause acoustic waves to be generated (hereafter, such an acoustic wave will be referred to as a photoacoustic wave). The photoacoustic waves are received by a transducer, and received signals are analyzed by a processing apparatus. As a result, information in terms of optical characteristic values of internal portions of the subject is obtained, and an optical characteristic value distribution in the inside of the subject is visualized.
By illuminating the subject with light having different wavelengths, it is possible to obtain a distribution of a concentration of a substance existing in the subject. In this case, more specifically, an optical coefficient (absorption coefficient μa) of an internal portion of the subject is determined for each wavelength, and then a distribution of a concentration of a substance is determined and visualized based on the substance-specific dependence of the optical coefficient on the wavelength. In particular, based on the concentration of oxyhemoglobin HbO and the concentration of deoxyhemoglobin Hb, it is possible to acquire the oxygen saturation in blood. In a case where two wavelengths are used, the oxygen saturation SO2 may be determined according to formula (1) described below.
                    [                  Math          .                                          ⁢          1                ]                                                                                                                                  SO                  2                                ⁡                                  (                  r                  )                                            =                            ⁢                                                [                                      HbO                    2                                    ]                                                                      [                                          HbO                      2                                        ]                                    +                                      [                    Hb                    ]                                                                                                                          =                            ⁢                                                                                                                                            μ                          a                                                      λ                            2                                                                          ⁡                                                  (                          r                          )                                                                                                                      μ                          a                                                      λ                            1                                                                          ⁡                                                  (                          r                          )                                                                                      ·                                          ɛ                      Hb                                              λ                        1                                                                              -                                      ɛ                    Hb                                          λ                      2                                                                                                            (                                                                  ɛ                        HbO                                                  λ                          2                                                                    -                                              ɛ                        Hb                                                  λ                          2                                                                                      )                                    -                                                                                                              μ                          a                                                      λ                            2                                                                          ⁡                                                  (                          r                          )                                                                                                                      μ                          a                                                      λ                            1                                                                          ⁡                                                  (                          r                          )                                                                                      ·                                          (                                                                        ɛ                          HbO                                                      λ                            1                                                                          -                                                  ɛ                          Hb                                                      λ                            1                                                                                              )                                                                                                                              (        1        )            
In formula (1), μaλ1 denotes an absorption coefficient at a wavelength λ1, and μaλ2 denotes an absorption coefficient at a wavelength λ2. Furthermore, εHb0λ1 denotes a molecular extinction coefficient of oxyhemoglobin at the wavelength λ1, and εHbλ1 denotes a molecular extinction coefficient of deoxyhemoglobin at the wavelength λ1. εHb0λ2 denotes a molecular extinction coefficient of oxyhemoglobin at the wavelength λ2, and εHbλ2 denotes a molecular extinction coefficient of deoxyhemoglobin at the wavelength λ2. Note that values of εHb0λ1, εHbλ1, εHb0λ2, and εHbλ2 are known. Furthermore, r denotes a position coordinate. To determine the oxygen saturation in blood, as may be seen from formula (1), it is necessary to determine the ratio of the absorption coefficient between two wavelengths.
On the other hand, an initial sound pressure P0 of a photoacoustic wave generated by an absorber in the subject as a result of light absorption is given by a following formula.[Math.2]P0=Γ·μa·Φ  (2)where Γ denotes a Gruneisen coefficient which is given by a coefficient of volume expansion (β) times a sound speed (c) squared divided by a specific heat at constant pressure (Cp). Φ denotes an amount of light at a particular position (in a local region), which is an amount of light falling on an absorber and is also called a light fluence. It is possible to calculate the initial sound pressure (P0) using a reception signal (PA signal) output from a probe that receives the photoacoustic wave.
From formula (2), the value of the ratio of the absorption coefficient between two wavelengths is given as follows.
                    [                  Math          .                                          ⁢          3                ]                                                                                                μ              a                              λ                2                                      ⁡                          (              r              )                                                          μ              a                              λ                1                                      ⁡                          (              r              )                                      =                                            Φ                              λ                1                                      ·                          P              0                              λ                2                                                                        Φ                              λ                2                                      ·                          P              0                              λ                1                                                                        (        3        )            
To determine the value of the ratio of the absorption coefficient, as may be seen from formula (3), it is necessary to determine the amount of light Φλ1 at the wavelength λ1 and the amount of light Φλ2 at the wavelength λ2. In the apparatus disclosed in PTL 1, light emitted from the inside of a subject is measured using a photodiode, and an average optical coefficient of the subject is determined from the measured light, and then a distribution of an amount of light Φ in the inside of the subject is calculated from the optical coefficient.
In the case of a method such as that disclosed in PTL 1, to estimate the distribution of the amount of light Φ in the inside of a subject, a procedure of determining an optical coefficient of a subject is necessary, which needs complicated processing and/or complicated system configuration. In view of the above, the present invention provides a simpler method of acquiring information in terms of a concentration.